1. Field of the Invention
This invention relates to an engine ignition timing control system for outboard motors.
2. Description of the Prior Art
In an outboard motor, particularly small, portable-class outboard motor, its internal combustion engine is frequently not equipped with a self-starter but instead with a recoil starter comprising of a rope wrapped around a flywheel that the operator pulls to start the engine. In this case, the engine is often equipped with a decompression mechanism (device for automatically lowering starting load) in order to reduce the recoil load and improve operability.
As taught by Japanese Utility Model Publication No. Sho 63(1988)-2565, for instance, the decompression mechanism is typically composed chiefly of a weight attached to an engine camshaft, a valve release member attached to the weight that when in the operating position drives the intake/exhaust valves in the opening direction, and biasing means (spring) that biases the weight against the centrifugal force acting on the camshaft so as to locate the valve release member in the operating position. In other words, the decompression mechanism makes the pistons easier to move and reduces recoil load by opening the valves slightly when rotational speed is slow enough for the spring to overcome the centrifugal force.
Although the decompression mechanism reduces burden on the operator, its operating principle of opening the intake/exhaust valves, even though only slightly, degrades engine starting performance. When a decompression mechanism is provided, therefore, the practice is generally to also install a start throttle opener or to enrich the air-fuel ratio markedly at engine starting.
Provision of a start throttle opener is, however, disadvantageous because it adds to structural complexity and thus cost, and also degrades space utilization. On the other hand, marked enrichment of the air-fuel ratio at engine starting is undesirable because it impacts the environment by lowering fuel economy, degrading emission and increasing exhaust gas fume smell, and also because it shortens the service life of the ignition plugs and is liable to cause loss of idling speed stability.
An object of this invention is therefore to overcome the aforesaid drawbacks of the prior art by providing an engine ignition timing control system for outboard motors that can control ignition timing to enhance engine starting performance, without need for a start throttle opener or air-fuel ratio enrichment, and is structurally simple.
For realizing this object, a first aspect of this invention provides a system for controlling ignition timing for an internal combustion engine, at starting, of an outboard motor having a propeller connected to the engine to be rotated, the outboard motor being to be mounted on a boat such that the boat is propelled forward or backward, comprising: engine speed detecting means for detecting speed of the engine; ignition timing setting means for setting the ignition timing of the engine to a predetermined crank angle until the detected engine speed exceeds a first prescribed engine speed, and for setting the ignition timing of the engine to a value obtained by retrieving a predetermined characteristic by at least the detected engine speed after the detected engine speed exceeds the first prescribed engine speed; ignition timing adjusting means for adjusting the ignition timing in a direction advanced than the retrieved value until the detected engine speed becomes equal to or greater than a second prescribed speed after the detected engine speed has exceeded the first prescribed engine speed; and ignition means for igniting air-fuel mixture in the engine based on at least one of the set ignition timing and the adjusted ignition timing.